There are numerous processes in the chemical industry and related fields in which a gas is heated to a high temperature and then used as the heat transfer medium or treating agent in a subsequent operation. For example, such operations include thermal decomposition processes for manufacturing fuel gas, town gas, synthetic gas, or raw material gas usable for synthetizing chemicals, such as ethylene, propylene and acetylene, from petroleum hydrocarbons; alteration processes for natural gas; distillation of coal, wood and various agricultural products for manufacturing carbonized products; oxidation processes for oxidizing metal ores, coal benzene and naphthalene; reduction processes for reducing metal oxides and ores; and activation processes for activating carbonized products.
Normally such processes are carried out by gigantic equipment on a large scale. Consequently, the following conditions must be satisfied in order to carry out such processes economically:
1. the treating condition must be maintained constant, i.e., the temperature and composition of the treating system must be maintained constant so that uniform quality products can be manufactured from the treating system, PA1 2. easy and continuous operation, PA1 3. low operating costs, and PA1 4. simple and low cost equipment and maintenance. PA1 1. The heat-exchange method -- in which the petroleum hydrocarbon is contacted with a treating gas preheated to a prescribed temperature by use of a heat-exchanger in order to decompose the petroleum hydrocarbon. PA1 2. The partial combustion method -- in which the petroleum hydrocarbon is fed into a decomposition chamber together with steam and air such that a portion of the petroleum hydrocarbon is subjected to combustion for heating the remaining portion of the petroleum hydrocarbon and steam whereby the remaining portion is decomposed by the action of superheated steam.
However, all of the above requirements have not been successfully satisfied in the prior art methods and apparatuses in which raw material is treated with high temperature gas. Some of the numerous problems encountered in the previous methods and apparatus will be described hereinafter. For example, in the thermal decomposition process for decomposing petroleum hydrocarbon, the following methods are used in which high temperature gas functions as the heat transfer medium:
The former method has the disadvantage of necessitating a heat-exchanger, while the latter method has the two drawbacks of trying to maintain the combustion atmosphere in the decomposition furnace always constant and trying to prevent undesirable coke formation due to incomplete combustion of the petroleum hydrocarbon.
By way of further example, in the drying or distilling process for solid material, it is necessary to use a high temperature gas as the heat transfer medium which is substantially inert and does not oxidize or burn the solid material. Furthermore, this high temperature gas must always be produced at constant temperature and with uniform composition. However, it has not been possible for the prior art distillation or drying methods and apparatus to completely satisfy such requirements.
Furthermore, as another example, in the oxidizing process of coal or metal ores by means of a high temperature treating gas, it is necessary to accurately control the quantity and temperature of the oxygen or air used as the oxidizing agent, but these requirements have not been completely satisfied with the prior art methods and apparatus in which fuel is burnt after mixing with a large excess of air and the heat energy of the burnt gas is utilized as the heating source with the excess air in the burns gas being used as the oxidizing agent.
For still another example, in the process of reducing metal oxides or metal ores with a high temperature reducing gas, it is necessary to feed a reducing gas, such as carbon monoxide, to the reaction system at a constant temperature and concentration, but in order to prepare such a reducing gas, it has been necessary in the prior art to use a heat-exchanger and such renders the reducing process too expensive. Also, in the activated carbon manufacturing process, the conventional activation of the carbonized material is carried out in either of two techniques -- the carbonized material is either treated with a high temperature steam which has been superheated by preheating, or the carbonized material is treated with a mixed high temperature gas produced by the combustion of fuel mixed simultaneously with air and steam so that the mixed gas contains the steam superheated with the combustion heat. However, a special heat-exchanger is required for superheating the steam in one technique whereas in the latter technique, it is difficult to maintain constant the temperature of the mixed gas.